Development of Improved Heat Sources for a New Generation of Cold District Heating Systems

Abstract

Cold district heating systems offer the opportunity to utilize low temperature heat sources for the heat supply of buildings. They are often operated in combination with thermal energy stores and heat pumps. In this publication air-brine-collectors and an ice store are investigated experimentally to improve the heat transfer and therefore increase the efficiency of connected heat pumps due to higher source temperatures. Firstly, several investigations with an experimental ice store were performed. On the one hand, the heat transfer capacity of the heat exchanger in the ice store could be increased significantly in regeneration periods. On the other hand, the heat transfer capacity of the heat exchanger could also be increased by 62 % in extraction periods during dynamically alternating extraction and regeneration periods compared to continuous extraction without dynamical alternation. Secondly, the effect of ice formation around the heat exchanger pipes and fins of an air-brine-collector on the heat transfer was investigated. Therefore, a PVT air-brine-collector was installed in a climate chamber and operated under boundary conditions supporting ice formation by a factor of approximately 3 compared to boundary conditions at a real system in southern Germany. During 21 hours of ice formation the heat transfer capacity decreased by 36 % due to increasing thermal resistances caused by various influencing factors like ice formation and a decreasing volume flow rate. Based on analytical calculations of the thermal resistances it is assumed that limited air convection due to increased ice formation between the fins of the heat exchanger has the most significant influence on the heat transfer between heat transfer medium and ambient air.